Lesson 2 – Getting started with

Tip – check the clubs website periodically to see these lessons on-line as well as the library of CAD drawings that will be gradually be added to it. The first thing I was told when learning CAD was 'never draw the same thing twice'. That is why the CAD library exists. If I have drawn it, it saves you having to re-invent the wheel. There are handy items such as servos, engines, quick links etc. to name a few.

In the first lesson you were introduced to the commands line, circle, rectang, erase as well as the snap codes nea, end, mid and cen.
In this lesson you will be introduced to fewer but more complex commands.

Commands:

pline, regen, scale, rotate, offset, move as well as managing layers.
The first lesson may have been a bit of a case of 'so what!', I could do that with pain brush - right?

Any similarities to paint end with this lesson. CAD graphics are vectors not raster. That means real co-ordinates that can be moved, edited and scaled/rotated etc. far beyond what can ever be done with paint.

The pline command

This command literally stands for 'polyline', which in real English is that all lines drawn whilst in this command, are joined together. Lines drawn with the line command are all separate lines that have the same start and end points. Polylines start and end at the same co-ordinates, but if you try to move one of the lines, the entire group will move. This is particularly useful, as complex shapes can be moved by picking the group up at any point rather than selecting all items individually to move. Poly lines can and often do include arcs. Just the thing for wing ribs and fuselage outlines. Ordinary lines can be included into a poly line group.

This command is invoked with pline. The first point can be anywhere on the screen or a known point such as intersection, middle, centre or near etc. You will then be asked to insert the next point. This can be another known location, anywhere on the screen, included arc, a distance, hafwidth or width. Polyline also has the added advantage of being able to change the line width. That includes changing line width such that it tapers. A use for this is things such a arrows.

The regen command

Regen literally means regenerate. Regen and redraw are similar except that regen repaints the screen such that circles appear as real circles when
zoomed in for example.

Circle before regen.	Circle after regen.

The scale command

This is a particularly useful command. Wing ribs are a cinch. In the example below we are going to do just that. This is especially time saving
when dealing with tapered wings. Invoke scale and select the items you want to change the size of. Theirs can be done with 'one click, one item' or with a 'window' around a group. Then you are asked for a base point, select the end of the line touching the trailing edge. Next we are asked for a base scale or scale factor. Type in ref. This means reference. Select the end of the line touching the trailing edge. We are next asked for the second reference point. Select the end of the line touching the leading edge. Now as you move the mouse you will see a rubberbanding wing section. The final size can be decided upon by using known locations or a scale factor. Note that if at this point you enter 2 instead of a snap code, the rib will be scaled by 2 ie. Half, not 2X.

Wing rib before scale

Wing rib after scale

The rotate command

This command can be used for things such things as angles of incidence for main planes as well as engine offsets. Rotate operates in a similar fashion to scale. The objects are selected by selecting individual items or multiples with a window. In this example an Irvine 25 from our official SEMAC CAD library will be rotated clockwise by a given value. Invoke rotate and select the items with a window. We are then asked to provide a rotation point. Select the intersection of the centre line and the backplate. Next enter -5. The engine has just been rotated a factor of 5 degrees in the clockwise direction. The rotation point was the centre of the backplate.

Before rotate	After rotate

The offset command

Most of the commands do what is fairly obvious, and offset is no exception. This one is invaluable for things such as wing spars, leading edge and trailing edge plans. Lines, circles, polylines etc. can all generally be offset. Draw a line, then invoke offset. You will be prompted to enter a distance. Type 0.3 then enter. Select the line and click towards the side you wish the new line to appear at. You will be asked to select the next entity. You could select other objects to be offset at our pre-defined 0.3 or end the command with enter.

Line before offset	Line after offset

The move command

Move does exactly what you would assume. As with other commands, single items or groups can be moved. In this example, we will move out trusty Irvine 25 to the correct location on our engine bearers. Start move command, select with a window around the engine such that all items are included, if no more items are to be included in this move, hit enter and move from the lower right hand corner of the left engine lug (against the crank case) with an int or end snap, and move to somewhere near the engine bearer. Put it down with a perpendicular or any relevant snap code.

Before move	After move

Managing layers

Layers are an essential part of any CAD system. Layers can be compared to several acetate sheets being showed on an overhead projector.
They can be added and removed from the display, so you see the details you want to see and work with and hide the ones that may hold temporary graphics or anything not needing to be displayed at any moment. To bring up the layers dialog, IntelliCAD has the command explayers which means explore layers. An equivalent command in AutoCAD is ddlmodes of which I can think of no logical reason why they would call an essential operation with a command that may as well be a sample of alphabet soup !!! Either of these can be used to display the layers dialog. From here, you can add, remove and rename layers. Other things available here are important things such as assigning line types to specific layers. You may have a layer for centre lines. If this is the case you would assign 'center' as the linetype for that layer.(Note American spelling). Inside layers, you can also make some layers visible, invisible, locked (un editable) as well as change the colour. These are just the basics and are more than enough for most drawings without going to too much detail. When working with layers, you need to nominate which one is 'current'. If you want to add graphics to a layer called 'dimensions' for example you need to have created the layer, changed its colour and/or linetype, and set it to be the 'current' layer. That way, new graphics drawn from that point will be added to the layer we just nominated rather than having dimensions on the 'wheels' layer for example. To make a layer, click the layer name and click the icon with a picture of a tick in it. The tool tips will help here.